200 Arthur Wili.ey, 



The characteristic arrangement which distinguishes the gravid 

 uterus of the beaver from that of other Rodeuts is rendered more 

 intelligible by a diagram thau by a verbal description (Fig. B). 

 Across the apertures of the inter-utricular segments there is stretched, 

 in each case, a membrane which I will call the placento-uterine 

 membrane, since it extends between the stalk of the placenta 

 and the wall of the uterus. This membrane is made up of two 

 parts which meet each other and the vascular chorion atthesinus 

 terminalis. The latter therefore really marks the line of union 

 of three membranes, viz. : the vascular chorion; the umbilico- 

 placental membrane between the sinus terminalis and the 

 placenta; and the umbilico-uterine membrane between the 

 chorion and the uterine wall. The two latter taken together con- 

 stitute the placento-uterine membrane. 



The umbilico-uterine membrane effects a concrescence 

 between the chorion and the uterine mucosa. It is a thin trans- 

 lucent sheet of tissue about 5 mm in width round the ends of the 

 blactocyst, becoming reduced to zero as it reaches the stalk of the 

 placenta. In its position relative to the blastocyst. it corresponds 

 with the residual zone described by Duval in the rabbifs 

 blastocyst after the resorption of the distal wall of the umbilical 

 vesicle. This forms a narrow fold with a free edge encircling the 

 rabbit's blastocyst at the level of the sinus terminalis; but it 

 contracts no relations with the uterine wall. It remains to be de- 

 cided, by the acquisition of earlier stages. exactly how the con- 

 crescence is brought about in the beaver. At present we can only 

 deal with the definitive formation. 



The u m b i 1 i c o - p 1 a c e n t a 1 m e m b r a n e represents what there 

 is of the somatopleure of the exocoelom; the rest of the chorion is 

 the inverted vascular splanchnopleure of the exocoelom. The mem- 

 brane in question is transparent and, in general, non-vascular; it 

 forms a great part of the roof or placental wall of the blastocyst, 

 the middle portion of the roof being occupied by the placenta. The 

 distal or obplacental wall of the blastocyst comprises the whole of 

 the vascular chorion below the sinus terminalis. 



By cutting away the antimesometric Avall of the utricle and 

 leaving the mesometric wall with its foetal attachments intact, the 

 umbilico-uterine membrane is seen stretched between chorion and 

 mucosa in front of and behind the placenta (Fig. C). At this 

 moment of the examination, the placenta is feit as a hard body 



